The present invention relates to a braking system having a brake-assisting mechanism that assists a driver in his/her braking operation in an emergency to produce an increased braking force.
One example of conventional braking systems having a brake-assisting mechanism is disclosed in Japanese Patent Application Unexamined Publication Number [hereinafter referred to as "JP(A)"] 61-268560.
The above-mentioned conventional braking system has a brake-assisting mechanism incorporated in a brake booster.
The booster is arranged as follows. When the brake pedal is depressed, a judgment is made as to whether or not there is an emergency situation in which the driver wants to stop or decelerate the vehicle immediately in order to avoid an accident, for example, on the basis of an output from a sensor that detects the speed at which the brake pedal is depressed. Under normal circumstances, a vacuum valve is closed by an input rod that moves in accordance with the pedal pressure applied to the brake pedal by the driver. At the same time, an atmospheric valve is opened by the movement of the input rod to supply a working fluid into a variable-pressure chamber, as in the case of the conventional system, thereby causing a differential pressure to act on a power piston to obtain a booster action. Under emergency conditions where the driver wants to stop or decelerate the vehicle immediately in order to avoid an accident, for example, the brake booster operates as follows. In addition to the above-described supply of working fluid by the movement of the input rod, a control valve is opened to supply the working fluid into the variable-pressure chamber, thereby causing an even greater differential pressure to act on the power piston to obtain an enhanced booster action. Thus, the driver is assisted in his/her braking operation to produce an increased braking force.
FIG. 7 shows an example of the control operation of a conventional controller used in the above-described brake booster to open the control valve by judging an emergency situation on the basis of the brake pedal depressing speed.
In the illustrated example, the controller reads a stroke signal sB from a pedal stroke sensor (not shown) in a predetermined control cycle (step S1). At the subsequent step S2, the controller obtains a difference VB between the stroke signal value sB(n-1) read in the preceding control cycle and the stroke signal valve sB(n) read at step S1 in the present control cycle. The control cycle is started at intervals of a predetermined time by timer interrupt processing, for example. By the above-described difference computation, a speed (stroke speed) VB is calculated. That is, the controller calculates a stroke speed corresponding to the difference VB at step S2. In the following description, the stroke speed will be denoted by reference character VB appropriately.
At the subsequent step S3, the controller multiplies a reference value Seo of a stroke speed threshold value by a vehicle speed coefficient F1(V) and a stroke coefficient F2(sB) to calculate a stroke speed threshold value Se.
The vehicle speed coefficient F1(V) has been preset according to the vehicle speed as stated in JP(A) 7-76267, by way of example. More specifically, when the vehicle speed is low, the vehicle speed coefficient F1(V) is set at a high value to prevent the brake-assisting mechanism from coming into operation uselessly in response to a vigorous operation of the brake pedal during driving for parking or reversing. When the vehicle is running smoothly at a normal speed, the vehicle speed coefficient F1(V) is set lower than the coefficient set when the vehicle speed is low [the coefficient F1(V) includes a minimum value], thereby allowing the brake-assisting mechanism to be readily activated by a vigorous operation of the brake pedal. When the vehicle speed is very high, the vehicle speed coefficient F1(V) is set higher than in the case of the normal smooth running speed. The driver tends to actuate the brake pedal vigorously when the vehicle speed is very high. Therefore, the brake-assisting mechanism is prevented from coming into operation uselessly in response to the vigorous brake pedal operation.
The stroke coefficient F2(sB) is set as stated in JP(A) 7-76267 by way of example. The contents of this publication are hereby incorporated by reference. That is, the stroke coefficient F2(sB) lowers stepwisely as the stroke of the brake pedal increases. For example, as the brake pedal stroke increases, the reaction force of the booster increases. Therefore, the stroke coefficient F2(sB) is set to compensate for the disadvantage that, as the brake pedal stroke increases, it becomes more difficult for the driver to obtain a high brake pedal depressing speed, and hence more difficult to activate the brake-assisting mechanism.
At the subsequent step S4, brake stiffness monitor processing is executed to calculate a stiffness coefficient Kw.
The brake stiffness monitor processing is such as that shown in JP(A) 8-207721, by way of example. More specifically, monitoring is performed to detect a change in the reaction characteristics of the braking system due to a change in the elasticity of a diaphragm provided on the power piston of the booster or wear of the brake pads, for example, on the basis of the relationship between data such as the brake pedal stroke and the vehicle speed. When a change in the reaction characteristics is detected, a stiffness coefficient Kw is calculated to compensate for an influence due to the reaction characteristic change.
At the subsequent step S5, a driver's operating habit monitor processing is executed to calculate an operating habit coefficient.
The driver's operating habit monitor processing is such as that shown in JP(A) 7-156786, by way of example. The contents of this publication are hereby incorporated by reference. An operating habit coefficient is obtained as a correction coefficient concerning each individual driver. After completion of a braking operation conducted by each particular driver, an operating habit coefficient concerning the driver is calculated. More specifically, a characteristic coefficient K is calculated on the basis of a maximum stroke speed VBm and maximum stroke sBm of the brake pedal obtained during the braking operation. A preset model driver's operating habit coefficient (=1) is multiplied by the characteristic coefficient K to determine an operating habit coefficient unique to the driver concerned.
At the subsequent step S6, the stroke speed threshold value Se obtained at the previous step S3 is multiplied by the stiffness coefficient Kw and the operating habit coefficient to obtain a corrected stroke speed threshold value. Then, a comparison is made to judge whether or not the stroke speed VB obtained previously is greater than the corrected stroke speed threshold value, thereby deciding whether the present situation is an emergency or not.
If YES is the answer at step S6, that is, if the present situation is judged to be an emergency, the solenoid of the control valve is driven to open the valve. Consequently, the working fluid is supplied into the variable-pressure chamber from the working fluid source through the control valve in addition to the supply of working fluid into the variable-pressure chamber by the vacuum and atmospheric valves actuated by the movement of the input rod as in the case of the conventional system. By doing so, an even greater differential pressure is produced to act on the power piston to obtain a booster action, thereby assisting the driver in his/her braking operation to produce an increased braking force.
Then, the above-described control (subroutine) having steps S1 to S7 is terminated. This control operation is repeatedly executed in a predetermined control cycle.
In a braking system having a brake-assisting mechanism that assists the driver in his/her braking operation in an emergency to produce an increased braking force as stated above, the brake-assisting mechanism should be activated only in an emergency situation in which the operation of the brake-assisting mechanism is actually needed. It should not be activated in a situation of low emergency level.
For this reason, as stated in connection with FIG. 7, the setting of a stroke speed threshold value for a judgment as to whether or not the present situation is an emergency that needs the brake-assisting mechanism to be activated must be adjusted finely according to the vehicle speed and the brake pedal stroke. Furthermore, even when the same pedal pressure is applied to the brake pedal, the brake depressing speed may vary owing, for example, to a change in the elasticity of the diaphragm provided on the power piston of the booster or wear of the brake pads. Therefore, the stroke speed threshold value must be corrected by also monitoring the brake stiffness at all times.
Moreover, there are differences among individuals in the brake pedal pressure applied in an emergency (i.e. the way in which the brake pedal is actuated). In other words, the way in which one individual depresses the brake pedal in an emergency situation may be within the range of the way in which another individual depresses the brake pedal in a normal situation. Therefore, the stroke speed threshold value must be corrected according to each individual's operating habit.
Accordingly, the setting of coefficients for these correction operations is complicated and requires preliminary studies to be conducted on a large number of drivers at much expense in time and effort.
Even if the stroke speed threshold value is corrected strictly by expending much effort, the way in which the brake becomes effective in response to the actuation of the brake pedal may change greatly at a stroke speed around the corrected stroke speed threshold value. This may confuse the driver's sense of controlling the brake pedal.